But glob­ally, there’s no “ro­bust” ev­i­dence for a glob­al in­crease in jel­ly­fish over the past two cen­turies, ac­cord­ing to a study pub­lished in the lat­est is­sue of the jour­nal
Pro­ceed­ings of the Na­t­ional Acad­e­my of Sci­ences.

Cathy Lu­cas of the Uni­vers­ity of South­amp­ton in the U.K., co-au­thor of the stu­dy, said that al­though there is enough cause for con­cern to keep mon­i­tor­ing the situa­t­ion, some re­cent me­dia re­ports of jel­ly­fish popula­t­ion in­creases have been overstated.

The cur­rent mea­sure­ments show a “weak in­creas­ing lin­ear trend in jel­ly­fish popula­t­ions af­ter 1970,” Lu­cas said, and a growth in jelly popula­t­ions dur­ing the 1990s and early 2000s that was, how­ev­er, cy­clic. Jel­ly­fish popula­t­ions rise and fall eve­ry 20 years or so, ac­cord­ing to new find­ings by the group.

“Sus­tained mon­i­tor­ing is now re­quired over the next dec­ade” to see wheth­er re­cent in­creases are part of a larg­er trend, Lu­cas said. In some ar­eas, she added, jel­ly­fish popula­t­ions have de­creased.

“There are ma­jor con­se­quenc­es for get­ting the an­swer cor­rect for tour­ism, fish­er­ies and man­age­ment de­ci­sions as they re­late to cli­mate change and chang­ing ocean en­vi­ron­ments,” said Lu­cas. “The im­por­tant as­pect about our work is that we have pro­vid­ed the long-term base­line backed with all da­ta avail­a­ble to sci­ence, which will en­a­ble sci­en­tists to build on and eventually re­peat these anal­y­ses in a dec­ade or two from now to de­ter­mine wheth­er there has been a real in­crease in jel­ly­fish.”

“The realiza­t­ion that jel­ly­fish syn­chro­nously rise and fall around the world should now lead re­search­ers to search for the long-term nat­u­ral and cli­mate drivers of jel­ly­fish popula­t­ions,” added Rob Con­don, a ma­rine sci­ent­ist at the Dau­phin Is­land Sea Lab in Al­a­bama and lead au­thor of the stu­dy.

Humans and their pollution are creating many problems in the ocean—but a global jellyfish boom does not seem to be among them for now, scientists say.
Local “blooms,” or proliferations, of jellyfish can have nasty effects on coastal communities: clogged nets for fishermen, stinging waters for tourists, even choked cooling intake pipes for power plants.
But globally, there’s no “robust” evidence for a global increase in jellyfish over the past two centuries, according to a study published in the latest issue of the journal Proceedings of the National Academy of Sciences.
Cathy Lucas of the University of Southampton in the U.K., co-author of the study, said that although there is enough cause for concern to keep monitoring the situation, some recent media reports of jellyfish population increases have been overstated.
The current measurements show a “weak increasing linear trend in jellyfish populations after 1970,” Lucas said, and a growth in jelly populations during the 1990s and early 2000s that was, however, cyclic. Jellyfish populations rise and fall every 20 years or so, according to new findings by the group.
“Sustained monitoring is now required over the next decade” to see whether recent increases are part of a larger trend, Lucas said. In some areas, she added, jellyfish populations have decreased.
“There are major consequences for getting the answer correct for tourism, fisheries and management decisions as they relate to climate change and changing ocean environments,” said Lucas. “The important aspect about our work is that we have provided the long-term baseline backed with all data available to science, which will enable scientists to build on and eventually repeat these analyses in a decade or two from now to determine whether there has been a real increase in jellyfish.”
“The realisation that jellyfish synchronously rise and fall around the world should now lead researchers to search for the long-term natural and climate drivers of jellyfish populations,” added Rob Condon, a marine scientist at the Dauphin Island Sea Lab in Alabama and lead author of the study.